Nickel laterite ores are oxidized or weathered ores that typically occur in tropical areas where there has been an opportunity for mineralogical transformation from the original mineral deposit to a lateritic profile. Nickel laterites are expected to be an increasing source of nickel for the world market due to inadequate resources of nickel sulphide deposits.
The nickel laterite deposits fall broadly into two categories, limonite laterite deposits and saprolite laterite deposits. The limonite deposits have nickel contained substantially within an iron oxy-hydroxide mineral called goethite. The recovery of nickel from these limonite ores usually requires treatment of the whole ore using hydrometallurgy (although in some cases physical upgrading may be possible), possibly after an initial pyrometallurgical treatment. The direct hydrometallurgy route generally applied is high pressure acid leaching (HPAL). This process generally uses ˜245-270° C. autoclave leaching of the limonite with sulphuric acid to dissolve the goethite and re-precipitate hematite. The nickel and cobalt contained within the goethite mineral are released during the high pressure acid leach and subsequently recovered from solution following a solid-liquid separation, solution purification and recovery of a nickel product (for example, nickel metal cathode, nickel metal or nickel oxide powder, mixed hydroxide precipitate or mixed sulphide precipitate). An alternative process is the Caron process which first subjects the limonite to a reduction roast process to recrystallize the goethite to hematite and reduce nickel, cobalt and some iron to the metallic state. The calcine from the reduction roast is then oxidatively leached in a solution of ammonia-ammonium carbonate to dissolve nickel and cobalt. This solution then advances to metal recovery for nickel and cobalt.
Saprolite laterite deposits contain basic minerals that are concentrated in magnesium silicates. Magnesium silicates consume excessive amounts of acid in HPAL. The Caron process may be used for saprolites but recovery is generally low. Commonly, smelting is applied to saprolites to produce either a Fe—Ni (ferro nickel) product or a Ni—Co—S matte. The ferro nickel product can be used to make stainless steel and the nickel-cobalt-sulphur matte can be refined to nickel and cobalt metal.
In general, processes involving sulphation require the use of aqueous or concentrated sulphuric acid or can be conducted with sulphurous gases (sulphur dioxide and sulphur trioxide) as the sulphating agent (Canterford, J. H. (1975), “The Treatment of Nickelferous Laterites”, Minerals Sci. and Eng. 7: 3-17 (Canterford)). Alternatively, sulphuric acid may be produced “in-situ” by oxidation of sulphur, pyrite or other metal sulphides (CA922,903). This process chemistry combines high temperature oxidation of sulphides with high temperature leaching of laterite ores. In many instances, the leach liquors that are produced by sulphation processes are highly contaminated, and there are accordingly many recovery procedures that may be used to isolate nickel and cobalt or to remove contaminants. For example, dissolved iron may be removed by using either a jarosite process or a goethite process. Other procedures involve the selective cementation of nickel by iron, or the selective precipitation of a high-grade sulphide product using activated pyrrhotite.
Atmospheric laterite leaching processes have also been described. The use of a high temperature autoclave pressure vessel may for example be avoided through the use of “pug roasting” in which the acid is added directly to the laterite feed followed by roasting (Canterford). Acid leaching under reflux conditions at atmospheric pressure has however been reported to result in the generation of a contaminated liquor. In addition, high acid concentrations and extended leaching times have been used in examples reporting high levels of recovery of the contained nickel and cobalt (Canterford). One problem with this process is the generation of waste liquors that may be difficult to treat. For example, after nickel and cobalt recovery, an acid ferric sulphate solution may result, and the disposal of this liquor may be problematic (Canterford).
In an atmospheric leaching treatment, as for example disclosed in U.S. Pat. No. 6,261,527, the leaching reactions for limonite leaching can be specified as follows:2FeO(OH)+3H2SO4═Fe2(SO4)3+4H2ONiO+H2SO4═NiSO4+H2OCoO+H2SO4═CoSO4+H2OMgO+H2SO4═MgSO4+H2OZnO+H2SO4═ZnSO4+H2OCuO+H2SO4═CuSO4+H2OAl2O3+3H2SO4═Al2(SO4)3+3H2OCr2O3+3H2SO4═Cr2(SO4)3+3H2OMnO+H2SO4═MnSO4+H2OMnO2+SO2═MnSO4 
The result of atmospheric leaching of limonites is generally the dissolution of large amounts of iron, aluminum and other elements along with nickel and cobalt, hence the importance of the saprolite leaching/jarosite precipitation step. Saprolite leaching consumes excess acid and drives the precipitation of jarosite or goethite precipitates.
For the purpose of writing saprolite reactions, the “serpentine” mineral may be represented as a hydrated magnesium silicate Mg3[Si2O5](OH)4. Alkali cations are added to provide species for forming alunite and jarosite precipitates. For the purpose of writing reactions, the alkali cation will be provided by sodium chloride (NaCl) and sodium jarosite/alunite will be formed. The saprolite reactions can be specified as follows:Mg3[Si2O5](OH)4+3H2SO4=3MgSO4+2SiO2+5H2O3Fe2(SO4)3+2H2O+2Mg3[Si2O5](OH)4+2NaCl=2NaFe3(SO4)2(OH)6+5MgSO4+4SiO2+MgCl2 3Al2(SO4)3+2H2O+2Mg3[Si2O5](OH)4+2NaCl=2NaAl3(SO4)2(OH)6+5MgSO4+4SiO2+MgCl2 
An alternative embodiment of atmospheric leaching, incorporating goethite precipitation, has been reported (Liu, H. et al. (2004), “Atmospheric Leaching of Laterites with Iron Precipitation as Goethite”, International Laterite Nickel Symposium—2004, Ed. Imrie, W. P., et al., TMS (Warrendale), pp. 347-368), in which no source of alkali cation is provided to drive the formation of jarosite and alunite.
A further alternative process involves direct leaching of highly serpentinized saprolite ores using high strength sulphuric acid in either fresh or seawater (U.S. Pat. Nos. 6,379,637; 6,391,089; and Curlook, W. (2004), “Improvement to the Acid Pressure Leaching of Nickel Laterite Ores”, International Laterite Nickel Symposium—2004, Ed. lmrie, W. P., et al, TMS (Warrendale), pp. 325-334).
There are accordingly a range of processes in which large amounts of acid are applied to extract a substantial quantity of nickel from nickel laterite ores.